Field of the Invention
The invention relates to a sheet guide device for a supply region of a sheet-fed machine, in particular a printing press, which guides the sheets after separation from a feeder stack.
When sheets in a stack of sheets are supplied to sheet-fed machines, it is conventional to use a suction feed device which includes lifting suction devices and drag suction devices. The separation of the topmost sheet from a stack is carried out by aspirating the sheet in a lower position with the lifting suction devices and then raising the lifting suction devices. While the sheet is in the raised position, blown air is passed beneath the sheet by separating blowers operating from the rear edge of the stack, in order to reinforce the separation of the sheet from the stack and the raising of the sheet, while in order to maintain the air pressure, lateral loosening blowers blow air continuously against the side edges of the stack in the leading portion of the sheet. Once the lifting suction devices have reached their highest position, horizontally reciprocating feed suction devices (drag suction devices) take over the sheet from the lifting suction devices in that they aspirate the sheets, and the suction at the lifting suction devices is turned off. Next, the feed suction devices transport the sheets by a horizontal motion in the direction of the sheet-fed machine, so that the leading portion of the sheet arrives at a transfer point where there are sheet transport devices of the downstream sheet-fed machine, such as conveyor belts guided through a feeder table. The takeover by the sheet transport device of the sheet-fed machine is effected by synchronized lowering of synchronizing rollers, as a result of which the sheet is pressed against the revolving conveyor belts. Simultaneously with the lowering of the synchronizing rollers, the supply of suction to the feed suction devices is discontinued, and the feed suction devices are moved to their rear position in terms of the sheet feeding direction. Since the sheet moves along the transport path away from the rear loosening blowers, air increasingly reaches the upper side of the sheet, causing the sheet to flutter. This threatens the stable, reliable transporting of the raised sheet, especially in the leading region of the sheet, over a specified transport path to the sheet-fed machine and thus threatens the arrival of the sheet at the transfer point that is meant to be adapted to the pace of the sheet-fed machine.
In order to solve that problem, sheet guide elements are used above the sheet travel path. One such sheet guide element is known from German Published, Non-Prosecuted Patent Application DE 17 86 232 A1. In the supply region of a sheet supply device disclosed therein, the sheet is guided on its top side along the sheet travel path above the sheet stack by a plurality of horizontally disposed sheet guide rods that can be lengthened in telescoping fashion. The telescoping rods extend above the sheet stack from a crossbar, that is located downstream of the suction head in the sheet feeding direction, is disposed crosswise to the sheet feeding direction and is connected to the suction head, horizontally in a single plane as far as the transfer point to a feeder table of a printing press. Upon a format-dependent adjustment of the suction head in or counter to the sheet feeding direction, the aforementioned sheet guide rods are compressed in telescoping fashion. The tubular portions of the sheet guide rods form stepwise-extending segments. The guide elements are essentially dimensionally stable. A substantial disadvantage is that the sheet guide elements require maintenance for the sake of their functional reliability, since the telescoping rods are exposed to increasing wear from soiling and abrasion.